Most internal combustion engines in automotive and heavy duty truck applications include an engine cooling system operable to transfer excess heat generated by the engine to ambient. Such systems typically circulate a coolant fluid through the engine and through a radiator situated near the front of the engine. As the vehicle housing the engine is driven, air flows through the porous radiator and transfers excess heat to ambient. In certain vehicle operating conditions, however, the amount of air flowing through the radiator due strictly to vehicle velocity (typically referred to as "RAM air") is insufficient to transfer all of the excess heat from the coolant fluid. Consequently, most engine cooling systems include an additional air flow device situated between the engine and radiator, wherein the air flow device is controllable to provide additional air flow through the radiator. Typical air flow devices are embodied as one or more engine cooling fans which may be controllably driven by the engine itself or via a separate motor.
Known engine cooling fan control systems rely on one or more sensor signals, indicative of various engine/vehicle operating conditions, to control fan operation. For example, U.S. Pat. No. 4,313,402 to Lehnhoff et al. discloses an engine fan control system wherein the average fan speed is controlled to be proportional to engine speed when coolant temperature and engine speed are within specified ranges. U.S. Pat. No. 4,651,966 to Noba discloses a similar engine fan control system including provisions for controlling fan operation as a function of air conditioning load, and wherein two such fans are controlled independently to achieve a desired result. U.S. Pat. No. 5,609,125 to Ninomiya discloses another engine fan control system responsive to coolant fluid temperature and the rate of change of coolant fluid temperature to correspondingly control fan operation. Finally, U.S. Pat. No. 5,359,969 to Dickrell et al. discloses an intermittent engine fan control system wherein fan operation is based on engine speed, coolant temperature, intake manifold air temperature, boost pressure and engine brake status.
While the foregoing systems have been generally successful at controlling engine fan operation as needed based on the various engine/vehicle sensor inputs, it is generally known that engine fan operation is parasitic in that it consumes engine horsepower rather than contributing to it. It has accordingly been recognized that the overall efficiency of the engine can be increased by disengaging engine fan operation when it is not absolutely essential for maintaining engine temperature within a desired range of normal operating temperatures. The aforementioned Dickrell et al. system achieves this goal by basing fan activation events on the various sensor signal values. However, the Dickrell et al. system also suffers from certain drawbacks. For example, the Dickrell et al. system is only operable to deactivate the fan when it is not needed, and while increased fuel economy can accordingly be realized with this system, engine efficiency cannot be fully optimized. What is therefore needed is an engine fan control system that not only increases fuel economy but also optimizes overall engine operational efficiency. Such a system should further preferably achieve other vehicle operational benefits, such as controlling downhill vehicle speed, improving transient response and reducing fan noise during idle and low vehicle speeds.